This invention relates generally to a rotary fluid power transmission in the form of a pump or motor, and more particularly concerns a variable displacement gerotor pump.
It is well known that conventional gerotor pumps are positive displacement pumps that are self-priming, lightweight and require no valves for operation. Gerotor pumps have long been used to pump impure fluids and are durable, long-wearing devices.
A conventional gerotor pump includes two pumping elements, referred to herein as an inner rotor and an outer rotor. The inner rotor is generally secured to a drive shaft and always has one less tooth than the outer rotor. As the inner rotor is rotated on the drive shaft, it advances one tooth space per revolution relative to the outer rotor. The outer rotor is rotatably retained in a housing, eccentric to the inner rotor, and meshing with the inner rotor on one side. As the inner and outer rotors turn from their meshing point the space between the teeth of the inner and outer rotors gradually increases in size through the first 180.degree. rotation of the inner rotor, creating a partial vacuum therebetween. The fluid to be pumped is drawn from an inlet port into the enlarging space. During the last half of the revolution cycle, the space between the inner and outer rotors decreases in size as the teeth mesh and the fluid is forced from the space. As the space between the inner and outer rotors decreases in volume, it is open to an outlet port. The inlet and outlet ports are isolated from each other by the housing and the inner and outer rotors.
Such gerotor pumps are constant displacement pumps which yield a predetermined displacement per revolution. In many applications this is a desirable feature, however, in some applications it is desirable to change displacement without altering the speed of rotation of the drive shaft with a variable displacement pump.
While the advantages of a variable displacement pump are well known, prior art devices that have attempted to provide such a pump tend to be complex structures that are difficult to manufacture and subject to leakage or even failure. The degree of variability realizable in prior art gerotor pumps is severely limited. In particular, prior art variable displacement gerotor pumps are complex and are not generally as effective as other types of variable displacement pumps.
Conventional variable delivery gerotor pumps typically use a bypass to divert a portion of the fluid pumped from the fluid output channel of the pump to the reservoir or intake of the pump. The fluid may be either moved through a bypass channel or will flow internally from the outlet side of the pump to the inlet side. When variability is obtained by means of bypass, there is little power savings since the power requirement of the pump will remain the same even as delivery is reduced. If the fluid is permitted to flow internally from the outlet side of the pump to the inlet side, power is converted into heat which will build up in the fluid and pump, and will result in reduced service life of the fluid and pump. Many bypass systems also cause cavitation.
Another type of prior art mechanism used to create a variable delivery gerotor pump did so by restricting the outlet port of the pump. In this approach to the problem, restrictions in the outlet port result in excessive noise and vibration in the pump. Restricting the outlet causes fluid to be trapped in the pump which contravenes the traditional principles of fluid power engineering. Excessive noise and vibration in such devices is unacceptable in many applications and frequently will result in accelerated wear.
These and other disadvantages and limitations have been overcome in the present invention. While the simple and effective displacement control achieved by the present invention is best applied to gerotor pumps, it can also be applied to gear pumps, vane pumps, and other types of fluid rotary power transmissions including fluid rotary motors.